Currently used techniques for color projection displays tend to be relatively inefficient in their light utilization. Such low efficiency limits the brightness of the display, which in effect limits the acceptable amount of ambient lighting in a viewing environment.
In certain presently used designs, light from a spectrally broad source is collected by a condensing lens and illuminates a spatial light modulator system. The spatial light modulator system comprises a two-dimensional array of pixels and the amount of light transmitted through each pixel is controlled electronically. A projection lens then images the array of pixels on a viewing screen, the magnification of the displayed image being determined by the particular characteristics of the projection lens. The light impinging on each pixel of the spatial light modulator is spectrally broad (i.e., white light). Therefore, unless the system is modified to distinguish colors, the display is only capable of displaying black and white images.
In many current systems used to modify such a system so that it is capable of displaying color images, each pixel of the spatial light modulator is divided into three sub-pixels having equal areas. Each of the three sub-pixels is covered with a micro-color filter having a different spectral transmittance. For example, the filters are chosen such that one filter transmits only red light, another filter only green light, and the third filter only blue light. The transmittances of the three sub-pixels of each pixel of the spatial light modulator can be controlled independently, resulting in the ability to display a color image.
The inefficiency of the approach can be seen by considering the following factors. The light illuminating a full pixel essentially is white light and, consequently, the light impinging each sub-pixel is also white light. The red filtered sub-pixel transmits only red light, absorbing all of the incident green and blue light. Likewise, the other two sub-pixels transmit only its corresponding color, absorbing the other two colors. It is apparent that this approach utilizes, at most, only one third of the available light impinging on the modulator, and absorbs the rest.
Furthermore, state-of-the-art microcolor filters required to produce acceptable color images are approximately only 33% efficient in transmitting the color that they are designed to transmit. Therefore the overall light utilization of current color projection displays is about 10%.
One approach for improving the efficiency of color projection displays is found in U.S. Pat. No. 5,161,042 issued on Nov. 3, 1994 to H. Hamoda. In accordance therewith, the spectrally broad input light is supplied to three dichroic mirrors which reflect three different color components, e.g., red, green, and blue, in different directions, i.e., at different angles with respect to each other. The reflected components are then supplied to an array of lenses for focusing the different color components so as to converge light beams of similar wavelength ranges for transmission through a liquid crystal display element so as to form combined color images on a display screen. A further U.S. Pat. No. 5,264,880, issued on Nov. 23, 1993 to R. A. Sprague et al., discloses a similar approach to that of Hamoda wherein the dichroic mirrors are replaced by a phase grating for dispersing the color components of light received thereat into a spectrum of different colors at different angles relative to each other.
While such approaches can be used, the losses of energy of each color component are sufficient to reduce the efficiencies of such systems and to show the need for further improvement in such display systems. Such improved display systems should minimize such losses so as to provide for substantially the total use of the received energy across the color spectrum in the imaging display process resulting in an improvement of the efficiency of the system.